This invention relates to an amino group-protected 3,4-dihydroxyphenylalanine or a derivative thereof capable of chemically synthesizing 3,4-dihydroxyphenylalanine-containing peptides, and a process for producing the same.
Adhesive proteins secreted from shells such as a mussel, a barnacle, etc., have strong adhesive strength against a low energy surface of polytetrafluoroethylene, and the like. One of constitutive amino acids rich in the adhesive proteins is 3,4-dihydroxyphenyalanine (hereinafter referred to as "dopa"), which seems to have a close relation to adhesive mechanism.
Further, it is well known that dopa is a precursor of adrenaline and noradrenalin in higher animals, and is effective as a therapeutic drug for Parkinson's disease when used as a monomer. Therefore, peptides containing dopa are expected to have applications in medicines.
Thus, a technique for chemically synthesizing dopa-containing peptides in the same manner as for synthesizing normal amino acid-containing peptides has strongly been desired in order to develop novel adhesives and medicines.
In the synthesis of peptides using amino acids as starting materials by a chemical synthesizing method, it is necessary to protect functional groups such as an amino group or carboxyl group in the main chain of an amino acid, and/or an amino group, a carboxyl group or a hydroxyl group in a side chain with various protective groups, and if necessary removing (deblocking) the protective group from an intermediate, followed by participating in the reaction.
As a method for protecting an amino group in the main chain, there have been known a method of using a benzyloxycarbonyl group, a method of using a t-butyloxycarbonyl group, a method of using a 9-fluorenylmethyloxycarbonyl (Fmoc) group, etc. Among these methods, the method of using a Fmoc group has recently been noticed, since the deblocking can be carried out under mild basic conditions and chemical synthesis of peptides by a solid phase method is possible.
Protection of an amino group of some amino acids with a Fmoc group and application to peptide synthesis was first shown by L. A. Carpino et al (J. Org. Chem., 1972, 37, 3404-3409). Then, this was applied to a solid phase by J. Meienhofer et al (Int. J. Peptide Protein Res., 1979, 13, 35-42). C. Chang et. al. studied protection of an amino group with a Fmoc group in an amino acid wherein a side chain is protected with a t-butyl group, and physical properties of thus Fmoc-protected amino acid (the side chain being protected with the t-butyl group and the amino group being protected with the Fmoc group) (Int. J. Peptide Protein Res., 1980, 15, 59-66). I. Schon et al reported that 9-fluorenylmethyl pentalfluorophenyl carbonate was a useful reagent for the efficient, side reaction-free introduction of N-9-fluorenylmethyloxycarbonyl protecting group into amino acids and for the subsequent preparation of their pentafluorophenyl esters (Synthesis, 1986, 4, 303-305). R. C. Milton et al reported that 9-fluorenylmethyl succinimidyl carbonate (Fmoc-ONSu) was extremely useful in the rapid and efficient preparation of Fmoc-amino acids (Int. J. Peptide Protein Res., 1987, 30, 431-432).
Comparing the case of blocking an amino group of an amino acid having a functional group such as a hydroxyl group, an amino group, a carboxyl group at a side chain with a Fmoc group, with the case of blocking an amino acid of an amino acid having a functional group of side chain protected with a t-butyl group, a t-butyloxycarbonyl group, a benzyl group or a p-toluenesulfonyl group, etc. with a Fmoc group, when the amino acid is tyrosine, the yield is 70% in the case of the hydroxyl group at the side chain being not protected (see I. Shoen et al mentioned above), while the yield is 85% in the ease of the hydroxyl group at the side chain being protected with a dichlorobenzyl group (see R. C. Milton et al mentioned above), the yield of the latter being higher than the former.
Further, it is known that dopa having two hydroxyl groups at a side chain is unstable in neutral or alkaline, but when a borate complex is formed by reacting with boric acid or a borate in an aqueous solution, the resulting complex is remarkably stable in neutral or alkaline (Japanese patent Unexamined Publication No. 48-26745).
In the case of amino acid being dopa, it was impossible to produce dopa wherein the amino group is protected with a Fmoc group by any methods due to decomposition of dopa by an influence of basic conditions caused by sodium carbonate and sodium bicarbonate for attaching the Fmoc group. Thus, it was also impossible to easily produce peptides containing dopa in the molecule.